<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>demo3</title>
    <style>
        div {
            width: 300px;
            background-color: rgba(0, 0, 0, 0.7);
            color: #fff;
            font-family: 'Microsoft YaHei';
            text-align: center;
            padding: 5px;
            margin: 100px auto;
            position: relative
        }

        div::before {
            content: ' ';
            width: 0px;
            height: 0px;
            position: absolute;
            left: -12px;
            top: 50%;
            transform: translate(0px, -50%);
            border: 6px solid rgba(0, 0, 0, 0.7);
            border-color: transparent rgba(0, 0, 0, 0.7) transparent transparent
        }
         </style>
</head>
<body>
    <div>我是内容我是内容我是内容</div>
</body>
</html>